In AC to DC switch mode power supplies, the control circuitry is often powered by a bulk input storage capacitor during start-up, and then from an auxiliary winding on the transformer during steady state operation. The start-up circuitry, often called a "bootstrap" circuit, is intended to operate only during the transient start-up period, and must supply the control circuitry with enough energy to enable the power supply to reach steady state operation.
The energy stored in a capacitor is represented mathematically as 1/2CV.sup.2, where C is the capacitance value of the energy storage capacitor, and V is the voltage potential across the electrodes of the capacitor. As capacitance and voltage are inversely proportional relative to energy, in low voltage applications, the capacitance must be large to store enough energy to ensure proper power supply start-up. Large capacitors create several problems. First and foremost, they are expensive. Second, they are quite bulky. As power supplies become smaller and smaller, bulky parts constrain mechanical efficiency. Third, power supplies with bulky parts are hard to manufacture. There is therefore a need for an improved start up circuit that allows the sue of smaller capacitors.